It most likely means that there is no exact GPS data, but the company knows when trains arrive at a certain station and can calculate when the trains arrive at

It most likely means that there is no exact GPS data, but the company knows when trains arrive at a certain station and can calculate when the trains arrive at the next station (thus, they show us if a train or bus is late in minute-resolution). So it's easy to tell when a train is just departing, or when a train should be approximately 100 meters from the last station away, etc.